1. Field of the Invention
The present invention relates to a microwave detector for detecting microwaves emitted from measuring devices and the like, and in particular relates to a wideband microwave detector which uses a single receiver circuit to cover a wide frequency range.
2. Description of the Prior Art
Microwave detectors which generate an alarm upon detecting microwaves having the specific microwave frequencies emitted by radar type speed measurement devices are known in the prior art. Omitting a detailed description of the circuit construction, such microwave detectors generally operate by means of an antenna which picks up microwaves arriving from outside and a super-heterodyne type receiver circuit which receives microwave signals. Further, by sweeping the output frequency of the local oscillator in the related receiver circuit, such microwave detectors are able to ensure that the reception band width includes the microwave frequency of the detection target. Further, the local oscillator repeatedly carries out only a single sweep of the frequencies within the reception band width for each operation time period. Further, in the case where a voltage control type oscillator is used, such sweep can be carried out by outputting frequencies corresponding to the voltage which changes in a sawtooth pattern like that shown in FIG. 7.
At this time, if a microwave frequency within the reception band width is present, the receiver circuit outputs two peaks P within a prescribed time interval t, as shown in FIG. 8(a). Now, because the microwave frequency of the detection target is fixed for the time interval t, the presence or absence of a detection target microwave frequency can be determined by whether or not a pair of peaks P are present within the time interval t.
In reality, when no microwaves are present, a minute amount of white noise is outputted, and to deal with such white noise, a threshold value Th is established to create a margin that is sufficiently smaller than the level of the peak P, as shown in FIG. 8(a). Further, when a comparison between the output of the wave detector and the threshold value Th indicates that such output is below the threshold value Th, pulses are outputted (see FIG. 8(b)) and a judgement is carried out based on the time interval of such pulses. Further, such size relation comparison can be easily carried out by inputting the output of the wave detector and the fixed threshold value into a comparator.
In this connection, the performance of a microwave detector is determined primarily by its sensitivity. In this case, "sensitivity" refers to the ability to detect faint microwave signals.
In general, sensitivity is improved by using a high gain antenna and a high frequency circuit such as a low conversion loss mixer circuit, and by using a method which improves the performance of the devices used. However, random white noise is outputted in the wave detection output when no signals are present. Consequently, when such improvement techniques are employed, the white noise is amplified as well, and in the case where the reception level of the target microwave signal is small, the target microwave signal can be buried in the white noise, or the white noise can mistakenly be detected as the target microwave signal.
In particular, these problems associated with weak signals are common in the case where the emission source of the target microwave signal is far away, and this makes it difficult to carry out accurate detection. Namely, when the target microwave signal level is roughly the same as the white noise level, it becomes impossible to discriminate the target microwave signal and the white noise using the threshold value Th.
In other words, with prior art microwave detectors, it is difficult to detect faint microwave signals which have a level close to that of the white noise, and because this determines the detectable distance, the range of possible detection is limited.
On the other hand, in order to detect the target microwave signal as soon as possible for the benefit of the user, the detectable distance needs to be made as long as possible, and the detection of target microwave signals from far away emission sources needs to be carried out reliably. In order to solve such problem, a sweep of the same frequency range is repeatedly carried out, and by adding the detection outputs of each sweep cycle, it becomes possible to extract target microwave signals buried in white noise. Namely, because the white noise occurs at random, mutual cancellation will occur as the detection outputs from each sweep cycle are added together. However, because the target microwave signal possesses a specific frequency characteristic, the target microwave signal will be amplified as the detection outputs from each sweep cycle are added together. In this way, it is possible to extract microwave signals that are buried in white noise.
However, in order to carry out this operation, the detection output from each sweep cycle needs to be stored in memory, and then an adding process needs to be carried out. Moreover, in this arrangement the memory capacity must be capable of storing the entire detection output based on one sweep. Consequently, a large memory capacity is required, and this leads to high costs.
Furthermore, in such an arrangement where the entire frequency range is swept a plurality of times to obtain detected wave outputs which are then added together to extract the target microwave signal, the detection of the target microwave signal is not possible without a plurality of sweeps, and this results in poor responsibility.
Moreover, the detected wave outputs undergo signal processing after being converted to digital signals by an A/D converter. Accordingly, the use of a high resolution A/D converter is required in order to improve accuracy. In this regard, it is possible to detect a large amount of data within a shortened sampling time, but if the sampling time is shortened, the amount of data that is detected and stored as digital data is increased, and this further complicates the problems described above.